The present invention relates to a technique for selectively transmitting a radio wave having a predetermined frequency.
In recent years, the radio communication interface has spread in personal computers. A portable information device such as a notebook computer is carried freely and connected to a LAN environment into which the portable information device has been carried, by radio to conduct work. Such a use form has become widespread. However, the radio communication interface is easily affected by external electromagnetic noise. In addition, there is a problem of radio interference between radio communication interfaces and emission of a radio wave to the outside. Therefore, radio interference from external electromagnetic noise and emission of a radio wave are prevented by using a shield plate for shielding the radio wave.
If the shield plate is installed, however, the space is bisected via the shield plate to a region where a radio wave can be used and a region where a radio wave cannot be used. For preventing specific radio communication from being shielded, it is necessary to receive a radio wave once by using a receiver, pulls out the received signal across the shield plate by using a cable, and transmit the received signal as a radio wave again by using a transmitter.
As a technique for conducting radio communication between regions bisected via the shield plate without providing a transceiver and a cable, there is a technique disclosed in JP-A-2002-50893. However, this technique has a problem that the frequency selectivity of the antenna is low and the emission of the radio wave is significant in a frequency band higher than the radiant frequency of the antenna. Furthermore, it is necessary to form an antenna on only one side of the shield plate but also on the other side of the shield plate. Therefore, the structure for mounting the antenna portion on the shield plate becomes complicated, and the thickness of the shield plate increases, resulting in a complicated manufacturing process.
On the other hand, spread of the radio communication interface represented by IEEE 802.11b and Bluetooth® has advanced, and electronic devices, such as personal computers, incorporating these radio communication interfaces are increasing. In electronic devices such as computers, EMI (electromagnetic interference) emitted from the inside of the device is regulated, and it is typical to provide a device chassis with a shield structure so as to prevent EMI from being emitted from the inside of the device. In the case where a radio communication interface is incorporated in the chassis of the electronic device, therefore, conventionally a transmission and reception antenna is mounted outside the device, or a notch is provided in a shield structure of a portion incorporating the antenna so as to hold down the EMI to such a degree as to pass the regulation.
In a structure in which the antenna is mounted outside the device, the antenna portion projects. This results in a problem that the antenna is broken by falling of the device or a shock caused by touch. In the case where a notch is provided in the shield structure, there is a problem that the notch cannot be made large to minimize the EMI and the directivity of the antenna becomes narrow, or a problem that the antenna gain cannot be made sufficiently large.